1. Field of the Invention
The present invention relates to a mobile communication system for transmitting packet data in uplink. More particularly, the present invention relates to a method and apparatus for efficiently signaling control information which is used to control an uplink packet data service.
2. Description of the Related Art
A European-type 3rd generation mobile communication system, which is based on Global System for Mobile communication (GSM) and General Packet Radio Services (GPRS) and uses an asynchronous wideband code division multiple access (WCDMA), utilizes an enhanced uplink dedicated channel (E-DCH or EUDCH). The E-DCH has been proposed to improve the packet transmission performance in uplink communication of the asynchronous WCDMA communication system.
A mobile communication system supporting the E-DCH employs a Node B controlled scheduling scheme and a hybrid automatic retransmission request (HARQ) scheme in order to maximize the effectiveness of uplink transmission. According to the Node B controlled scheduling scheme, the statuses of the channels and the buffers of user equipments (UEs) are reported to a Node B, and then the Node B controls uplink transmissions of the UEs based on the reported information. The Node B allows a great amount of data to be transmitted to UEs having a good channel status, and minimizes the amount of data to be transmitted to UEs having a bad channel status, in order to efficiently utilize limited uplink transmission resources. According to the HARQ scheme, when an error occurs in a packet transmitted from a UE to a Node B, the packet is retransmitted in order to compensate for the error of the packet, thereby increasing the successful transmission rate as a function of transmission power. Through the HARQ scheme, a Node B does not discard a data block, in which an error has occurred during transmission of the data block, and soft-combines the data block having the error with a retransmitted data block, thereby increasing the probability of successfully receiving data blocks.
In an uplink, since orthogonality is not maintained between signals transmitted from a plurality of UEs, the uplink signals interfere with each other. As a Node B receives more uplink signals, interference with an uplink signal from a specific UE increases, thereby degrading the reception performance of the Node B. For this reason, the Node B restricts the number of uplink signals which can be received with its entire reception performance ensured. The radio resource of a Node B is expressed as shown in equation (1).Rot=Io/No  (1)
Herein, “Io” represents the total receiving wideband power spectral density of the Node B, and “No” represents the thermal-noise power spectral density of the Node B. Therefore, “ROT” represents uplink radio resources which the Node B can allocate for the E-DCH packet data service in an uplink.
FIGS. 1A and 1B are graphs illustrating changes in uplink radio resources which Node B is able to allocate.
As shown in FIGS. 1A and 1B, the uplink radio resources may be expressed as the sum of inter-cell interference (ICI), voice traffic, and E-DCH packet traffic. More specifically, FIG. 1A illustrates changes in the total ROT when the Node B-controlled scheduling is not used. In this case in which a scheduling for the E-DCH packet traffic is not performed, if a plurality of UEs may simultaneously transmit packet data at high data rates, the total ROT may exceed a target ROT, which degrades the reception performance of the uplink signals.
FIG 1B illustrates changes in the total ROT when the Node B-controlled scheduling is used. In this case of using the Node B-controlled scheduling, the Node B prevents a plurality of UEs from simultaneously transmitting packet data at high data rates. That is, according to the Node B-controlled scheduling, when a high data rate is allowed for a specific UE, low rates are allowed for other UEs, thereby preventing the total ROT from exceeding the target ROT.
As the data rate of a UE becomes higher, the Node B receives higher reception power from the UE, so that the ROT of the UE occupies more part of the total ROT of the Node B. In contrast, as the data rate of a UE becomes lower, the Node B receives lower reception power from the UE, so that the ROT of the UE occupies a smaller part of the total ROT of the Node B. The Node B performs the Node B-controlled scheduling for the E-DCH packet data, in consideration of the relationship between the data rate and radio resources and a data rate requested by the UE.
The Node B notifies each UE whether or not E-DCH data can be transmitted based on data rates requested from UEs using the E-DCH or based on channel status information, or performs the Node B-controlled scheduling to adjust the E-DCH data rates. The Node B-controlled scheduling is regarded as an operation that the Node B distributes the ROT to multiple UEs based on the statuses of the channels and the buffers of UEs performing E-DCH communication.
FIG. 2 illustrates a Node B and UEs which performs uplink packet transmission.
UEs 210, 212, 214, and 216 transmits uplink packet data at different uplink-channel transmission power levels 220,222, 224, 226 according to the distances between them and a Node B 200. The farthest UE 210 transmits packet data at the highest uplink-channel transmission power level 220, while the nearest UE 214 transmits packet data at the lowest uplink-channel transmission power level 224. The Node B 200 may schedule uplink data in a manner that makes the transmission power of the uplink channel inversely proportional to the data rate thereof in order to improve the performance of the mobile communication system, while maintaining the total ROT and reducing ICI. Therefore, the Node B 200 allocates relatively fewer transmission resources to the UE 210 having the highest uplink-channel transmission power, and allocates relatively more transmission resources to the UE 214 having the lowest uplink-channel transmission power, thereby efficiently managing the total ROT.
FIG. 3 is a view for illustrating a procedure in which a UE is allocated from a Node B with a transmission resource for transmission of E-DCH packet data, and transmits the packet data by using the allocated transmission resource.
An E-DCH is established between a Node B 300 and a UE 302 in step 310. Step 310 includes a step of transmitting/receiving messages through a dedicated transport channel. In step 312, the UE 302 transmits information about required transmission resources and scheduling information about an uplink channel status to the Node B 300. The scheduling information includes information about the uplink transmission power and transmission power margin of the UE 302, and buffer status.
The Node B 300 having received the information estimates the uplink channel status by comparing the uplink-channel transmission power with an actually-measured reception power. If the difference between the uplink-channel transmission power and the uplink-channel reception power is small, the uplink channel status is good. In contrast, if the difference between the transmission power and the reception power is large, the uplink channel status is bad. When the UE 302 transmits the transmission power margin, the Node B 300 estimates the uplink transmission power by subtracting the transmission power margin from a known maximum available transmission power of the UE 302. The Node B 300 determines an available transmission resource for an uplink packet channel of the UE 302, based on the estimated uplink transmission power of the UE 302 and information about the buffer status of the UE 302.
In step 314, the Node B 300 notifies the UE 302 of the determined transmission resource. In this case, the transmission resource may be the size of data which can be transmitted, that is, a data rate, or may be an available transmission power. The UE 302 determines the size of packet data to be transmitted through the reported transmission resource, and transmits data of the determined size to the Node B 300 in step 316. In this case, one unit of the packet data transmitted through an E-DCH is called a Media Access Control-enhanced Protocol Data Unit (MAC-e PDU).
As described above, buffer status information, and so on, required for providing an uplink packet data service through an E-DCH is essential control information for an efficient scheduling of a Node B. A protocol for transmitting/receiving control information as described above between a UE and a Node B is called a Medium Access Control for E-DCH (MAC-e). For this reason, the control information is called “MAC-e control information”. Therefore, a detailed method for more efficiently signaling the MAC-e control information has been required.